1. Field of the Invention
The present invention relates generally to the field of rotary blowers or compressors, and more particularly relates to a double rotor helical shaped multi-lobe type commonly known as rotary screw blowers or compressors, and more specifically relates to a shunt pulsation trap for reducing gas pulsations and induced vibration, noise and harshness (NVH), and improving compressor off-design efficiency without using a traditional serial pulsation dampener or a sliding valve.
2. Description of the Prior Art
A rotary screw compressor uses two helical screws, known as rotors, to compress the gas. In a dry running rotary screw compressor, a pair of timing gears ensures that the male and female rotors each maintain precise positions and clearances. In an oil-flooded rotary screw compressor, lubricating oil film fills the space between the rotors, both providing a hydraulic seal and transferring mechanical energy between the driving and driven rotor. Gas enters at the suction side and moves through the threads trapped as the screws rotate. Then the internal trapped volumes between the threads decrease and the gas is compressed. The gas exits at the end of the screws to a discharge dampener to finish the cycle. It is essentially a positive displacement mechanism but using rotary screws instead of reciprocating motion so that displacement speed can be much higher. The result is a more continuous and smoother stream of flow with a more compact size and replacing the traditional reciprocating types.
It has long been known that screw compressors inherently generate gas pulsations with pocket passing frequency at discharge, and the pulsation amplitudes are especially significant under high pressure or for operating conditions of either under-compression or over-compression as being observed in gas transmission or AC and Refrigeration applications. An under-compression happens when the pressure at the discharge opening is greater than the pressure of the compressed gas within the rotor threads just before the opening. This results a rapid backflow of the gas into the threads, a pulsed flow in nature, according to the conventional theory. All fixed pressure ratio compressors suffer from under-compression due to varying system back pressure and a fixed design pressure. An extreme case is the Roots type blower where there is no internal compression at all, or the under-compression is 100% so that gas pulsation constantly exists and pulsation magnitude is directly proportional to pressure rise from blower inlet to outlet. On the other hand, an over-compression takes place when the pressure at the discharge opening is smaller than the pressure of the compressed gas within the rotor threads, causing a rapid forward flow of the gas into the discharge. These pulsations are periodic in nature and very harmful if left undampened that can induce severe vibrations and noise and potentially damage pipe lines and equipments downstream.
To overcome the problem, a large pulsation dampener consisting of a number of chokes and volumes commercially called reactive type, is usually required at the discharge side of a screw compressor to dampen the gas borne pulsations. It is generally very effective in gas pulsation control with a reduction of 20-40 dB but is large in size and causes other problems like inducing more noises due to additional vibrating surfaces, or sometimes induces dampener structure fatigue failures that could result in catastrophic damages to downstream components and equipments. At the same time, discharge dampeners used today create high pressure losses that contribute to poor compressor overall efficiency. For this reason, screw compressors are often cited unfavorably with high gas pulsations, high NVH and low off-design efficiency when compared with dynamic types like the centrifugal compressor.
In addition to the commonly used serial dampening, various other methods, such as skewed porting or using Helmholtz resonators at discharge, have also been attempted throughout the years but with only limited successes. Among the published methods, a flow equalizing strategy is most widely used, for example, as first disclosed in U.S. Pat. No. 4,215,977 to Weatherston, and later in U.S. Pat. No. 5,051,077 to Yanagisawa (Ebara). The idea, say for under-compression as an example, is to feed back a portion of the outlet gas through a skewed discharge opening or a pre-opening port to the compressor chamber prior to discharging to the outlet, thereby gradually increasing the gas pressure inside the cavity, hence reducing discharge pressure spikes when compared with a sudden opening at discharge. However, its effectiveness for gas pulsation attenuation is limited in practice, only achieves 5-10 dB reduction, not enough to eliminate discharge dampener. Moreover, at the off-design conditions, say either an under-compression or an over-compression, compressor efficiency suffers too. The traditional method is to use a sliding valve so that internal volume ratio or compression ratio can be adjusted to meet different system pressure requirements. These systems typically are very complicated structurally with high cost and low reliability.
It is against this background that prompts the present invention to use a different approach based on a new gas pulsation theory that a combination of large amplitude waves and induced fluid flow are the primary cause of high gas-borne pulsations and low efficiency under off-design conditions.
The new gas pulsation theory is based on a well studied physical phenomenon as occurs in a classical shock tube (invented in 1899) where a diaphragm separating a region of high-pressure gas from a region of low-pressure gas inside a closed tube. As shown in FIG. 1a-1b, when the diaphragm is suddenly broken, a series of high amplitude expansion waves is generated propagating from the low-pressure to the high-pressure region at the speed of sound, and simultaneously a series of high amplitude pressure waves which quickly coalesces into a shockwave is propagating from the high-pressure to the low-pressure region at a speed faster than the speed of sound. An interface, also referred to as the contact surface that separates low and high pressure gases, follows at a lower velocity after the lead wave. Further compression is achieved by the reflected shock wave at the end wall of the low pressure region to the level very close to the initial high pressure. By analogy, the sudden opening of the diaphragm separating high and low pressure is just like the sudden opening of compression cell to discharge gas at off-design conditions.
To understand gas pulsation generation mechanism in light of the shock tube theory, let's review a cycle of a classical screw compressor as illustrated from FIGS. 2a to 2e by following one flow cell marked dark in a typical 4×6 lobe configuration. In FIGS. 2a, low pressure gas first enters the spaces between lobes of a pair of rotors axially as they are open to inlet during their outward rotation from inlet to outlet. At lobe position shown in FIG. 2b, gas becomes trapped between two lobes and compressor inner casing as it is transported from inlet to outlet. It is then being compressed as the trapped volume between the threads decrease as shown in FIG. 2c. FIG. 2d shows the compressed gas is suddenly opened to the outlet and discharged. A serial dampener is then employed to attenuate pulsations generated in the gas stream as shown in FIG. 2e. 
According to the conventional backflow theory, a backflow would rush into the cell compressing the gas inside as soon as the cell is opened to the discharge as in case of under-compression. Since it is almost instantaneous and there is no volume change taking place inside the cell, the compression is regarded as a constant volume process, or iso-choric. After the compression, the rotors continue to move against this full pressure difference, meshing out the compressed gas to outlet chamber and return to inlet suction position to repeat the cycle.
However, according to the shock tube theory, the cell opening phase as shown in FIG. 2c resembling the diaphragm bursting of a shock tube as shown in FIG. 1 b would generate a series of shock wave, expansion waves and induced flow. The shock wave front sweeps through the low pressure gas inside the cell and compresses it at the same time at a speed faster than the speed of sound as in case of the under-compression. While for the over-compression, a fan of expansion waves would sweep through the high pressure gas inside the cell and expand it at the same time at the speed of sound. This results in an almost instantaneous adiabatic wave compression or expansion well before the induced flow interface (backflow as in conventional theory) could arrive because wave travels much faster than the fluid, as illustrated by the wave propagation pattern in FIG. 2f-2g. In this view, the pressure waves or shock waves are the primary driver for the compression as in case of under-compression while the backflow is simply an induced flow behind the shockwave after compression takes place.
In view of the new theory in case of an under-compression, as the shockwave travels to low pressure cell as shown in FIG. 2f, a simultaneously generated expansion front travels in the opposite direction causing rapid pressure reduction and inducing backflow down-stream. On the other hand for the case of an over-compression, as the expansion wave travels to high pressure cell as shown in FIG. 2g, a simultaneously generated pressure wave front travels in the opposite direction causing rapid pressure increase in the pipe and inducing forward flow down-stream. It is this fast changing pressure at wave front by the speed of sound drives the pulsating flow and is the source of gas pulsation for a screw compressor. Any effective pulsation control should address these fast travelling large amplitude waves and induced flow while minimizing losses at the same time.
Based on this view, having a pre-opening before discharge as suggested by Weatherston or Yanagisawa could reduce gas pulsations by elongating releasing time. However, it failed to recognize hence attenuate the simultaneously generated expansion or shock waves at the opening that eventually travel down-stream unblocked, causing high gas pulsations. Moreover, the prior art failed to address the high flow losses associated with the high induced velocity through the serial dampener or discharging process, resulting in a low compressor off-design efficiency.
Accordingly, it is always desirable to provide a new design and construction of a screw compressor that is capable of achieving high gas pulsation and NVH reduction at source and improving compressor off-design efficiency without externally connected silencer at discharge or using a sliding valve while being kept light in mass, compact in size and suitable for high efficiency, variable pressure ratio applications at the same time.